Boilover issue

[SandLake]

Hi, I have a Honeywell Smartfit S Plan system connected to a Grant Vortex 50/90 Condensing oil boiler (with a Riello RDB1 burner). I have no reason to believe that either piece of equiment is not working correctly but I do have a general problem which I am struggling to identify and fix.
On infrequent occasions the system boils over. I cannot replicate it and everything appears to be working as it should.
I believe that this occurs just as the system determines that a satisfactory temperature has been reached (not sure if it is HW or CH). At this point the automatic air vent just above the boiler goes mad and the overflow pipe starts to boil leading to the system venting into the expansion tank.
Not sure if it helps but I try to keep the programmers temperature setting as low as possible (if it is down to freezing outside it needs turned up and if it is milder outside then turn it down) as I think this minimises this boilover.
I have had plumbers look at the system but as the problem cannot be demonstrated to them they have not been able to diagnose it. The best they have come up with that I need a bypass valve which may indeed be correct even though I have 2 radiators without TRV's - This is not a specific solution to this problem.
I admit I am an amateur at this but I believe the issue is around pump over-run. Not even exactly sure what that is but for the pump to continue to run (and valve open) for a few minutes after satisfactory temperature has been reached would disperse the residual heat.
The Smartfit system does allow for 'Pump Over-Run Boiler' wiring and therefore operation but the Vortex boiler only appears to allow for basic wiring so that wiring option isn't applicable.

Does this sound in anyway familiar to you? Have you any recommendations as to how I go about fixing this issue?

Thanks

 

[SimonG]

Boiler stat or overheat or both faulty. Had it on a firebird.

 

[Dogby]

Long way round and not most cost effective, rewire the system with conventional valves in to the Honeywell pcb wiring centre which allows for wiring with a pump over run . Definitely seems like the over run issue though

 

[drip-dry]

Boiler thermostat out of calibration.
Requires replacing..by pass requires fitting
No trvs on rad no good when heating mot,valve closed,happens when heating and hot water reached full temp me thinks..

 

[SandLake]

The Honeywell Smartfit uses low voltage cabling and valves so not straightforward to replace - although an entirely new control system may be the best (and a costly) option.

I accept that the Boiler Stat and overheat might be the problem - don't particularly understand that but nothing new there!

I also accept that a bypass would be a good idea - but it is not fix for this problem simply a good idea and it may prevent this problem from occurring.

The 2 radiators that don't have TRV's are the bathroom which I just don't believe needs to be controlled and also the hallway where the programmer (containing the room stat) is located - that rad is controlled by the overall CH system temperature but it does mean that the CH is never closed down by TRV's.

Thanks for the replies

 

[SimonG]

Rule out the easy stuff first. Forget a bypass. May have a partial blockage in system.

Get the stats checked first and go from there.

 

[gassafe]

Maybe a poorly piped up cold feed & expansion tank

 

[lame plumber]

sounds more like poorly designed system or a failing pump rather than stat issue which should cut out if theres a problem

 

[Geyser]

sounds more like poorly designed system or a failing pump rather than stat issue which should cut out if theres a problem

I was thinking pump as well. Id test capacitor or sticking